Preserve constant array shape when spreading a union of constant arrays in array literals

[phpstan-bot]

Summary

When using the spread operator in array literals with a value that is a union of constant arrays (e.g. ...($flag ? ['key' => true] : [])), PHPStan was degrading the result to a general array type like non-empty-array<'key'|'other', bool> instead of preserving the precise array shape array{other: bool, key?: true}.

Changes

• Modified InitializerExprTypeResolver::getArrayType() in src/Reflection/InitializerExprTypeResolver.php to handle unions of multiple constant arrays when processing spread items
• Changed the condition from count($constantArrays) === 1 to count($constantArrays) > 0
• For the string-key path (PHP >= 8.1): collects all keys across all constant arrays in the union, determines optionality based on whether a key appears in all branches, and unions value types per key
• For the integer-key path: merges by position across all constant arrays, marking positions not present in all branches as optional
• Correctly updates $hasOffsetValueTypes tracking when merged spread keys overlap with previously-set keys

Root cause

In InitializerExprTypeResolver::getArrayType(), when a spread item's value was a union type like array{spread: true}|array{}, getConstantArrays() returned 2 constant arrays. The condition count($constantArrays) === 1 failed, causing the code to fall through to the general fallback that called $arrayBuilder->degradeToGeneralArray(), losing the array shape information entirely.

Analogous cases probed

• OversizedArrayBuilder: Uses $valueType instanceof ConstantArrayType (single type only) — affects only arrays with >256 items, a rare edge case. Not fixed here.
• FuncCallHandler arg unpacking ($callArg->unpack): Similar count($constantArrays) === 1 pattern for function call argument unpacking (used by array_push etc.). Different context with different semantics — not fixed here.
• Other count($constantArrays) === 1 sites (NodeScopeResolver foreach, ConstantArrayType list-ness, ArrayType truncation): Inspected and confirmed to be unrelated to array literal spreading.

Test

Added tests/PHPStan/Analyser/nsrt/bug-14708.php with 9 test functions covering:

• The exact reproduction from the issue (test1, test2, test3)
• Multiple optional keys from separate spreads
• Overlapping keys between spread branches
• Integer keys with union of different-length arrays
• All branches having the same keys (non-optional result)
• Three-branch union
• Integer-only key unions
• Empty vs non-empty union

Fixes phpstan/phpstan#14708